Compressors, such as axial and centrifugal compressors, are used in the treatment of many gaseous streams, such as the treatment of natural gas already discussed, and in the refining of crude oil, in many systems and arrangements.
Usually there is a vapour recycle or recirculation line around the compressor to avoid ‘surge’. Normally, the phenomenon of surge occurs in compressors at low volumetric flow rates, and hence limits the minimum capacity of a given compressor. In the operation of a compressor, as the system resistance is increased, the ratio of absolute pressures (the absolute pressure at the outlet of the compressor divided by the absolute pressure at the inlet of the compressor) generated by the compressor increases to overcome this resistance. As the system pressure increases, less flow can pass through the compressor, and this will continue up to the maximum pressure ratio of the compressor. The limit in the minimum flow to the compressor for a particular compressor speed is known as the surge point. Determining the surge points for different compressor speeds provides a surge line.
Below the surge line the outlet pressure exceeds that which the compressor is capable of delivering, causing a momentary backflow condition. During backflow the system resistance decreases, causing the back pressure to drop, enabling the compressor to deliver increased flow. If the opposition to flow downstream of the compressor is unchanged, peak pressure ratio delivery will again be approached and backflow observed, producing the cyclic condition known as surge. The operation of a compressor beyond the surge point can produce considerable mechanical damage due to vibration, noise, axial shaft movement and overheating.
U.S. Pat. No. 4,971,516 discloses a method and apparatus for operating compressors to avoid surges. The compressor is controlled by determining a controlled variable from (i) direct measurement of the rate of volumetric flow, Q, of gas through the compressor with an acoustic flow meter in combination with a standard flow meter and (ii) measurement of the speed, N, of the compressor. The controlled variable is the Q/N ratio. The measured Q/N ratio is used to operate a recycle valve in a bypass line when it nears a variable set point determined from the sonic velocity, vsd, below which surge can occur. U.S. Pat. No. 4,971,516 also suggests that the set point can also be determined from the compressor speed N.
One problem associated with this operating method is that the set point determined as function of the sonic velocity or compressor speed is not invariant for changes of compressor inlet conditions like e.g. ratio of specific heats. Changes to a specific heat ratio occur when the composition of the compressed gas changes or when completely different gasses are to be compressed. The latter situation may occur when the operational mode of a compressor changes, for instance in a liquefaction plant where the refrigerant gas to be compressed in normal operation may be propane, while maintenance may be carried out using nitrogen.
U.S. Pat. No. 4,971,516 attempts to solve this problem by using a variable set point which is either a function of compressor speed N or the speed of sound, vsd, in the compressor stream. However, both N and vsd vary with compressor conditions i.e. these parameters are not invariant to compressor inlet conditions. Consequently, a set point calculated in this manner may exhibit an error when the pressure at the compressor inlet, density at the compressor inlet and/or the ratio of specific heats at the compressor inlets deviates from those used in the design of the set point function. As a result, set point correction based of functions of either the compressor speed N or the speed of sound vsd are not invariant for all compressor operating conditions.